Please explain step by step a$)$ Calculate the net present value of the investment, which for the first three years brings a positive flow of PLN $2,600$ at the end of each year, while the following year brings one flow of PLN $1,800$ in mid$-$year $($in the mid of $4$th year$).$ The investment amount is PLN $7,500$ now. Assume an interest rate $($cost of capital$)$ of $8\%.[$Hint: $1\mathrm{.}08^3\mathrm{.}5=1\mathrm{.}3091](3$ points$)$

b$)$ Please also provide the discounted payback period for this investment. $(2$ points$)$

c$)$ Comment the results obtained in a$)$ and b$)$ and the entire investment. $(2$ points$)$

d$)$ What amount of investment would give the same NPV as in a$)$ if the investment would bring PLN $2,800$ annually for $3$ years $($only$).(3$ points$)$